A typical incandescent three-way lamp produces three levels of light intensity (i.e., low, medium, and high) using two lamp filaments within the same optical housing. The two filaments are typically of different wattages. For example, one lamp filament can be a low wattage filament, and the other filament can be a high wattage filament.
Conventionally, these two filaments are connected in parallel to the lamp base. The lamp base itself has two contacts and a neutral contact. Each of the filaments operates at full voltage when activated.
Proper installation of the three-way lamp is achieved by using a three-way lamp socket, which has three contacts instead of the usual two for a single filament lamp. This third contact is typically off center in the bottom of the socket, and makes contact with the second filament circuit.
The three-way lamp is controlled using a three-way switch, which itself has four positions. Starting from the ‘off’ position, the switch can sequentially connect power to one filament (typically the lower wattage filament,), then the other filament, and then both filaments.
A standard compact fluorescent lamp does not typically provide three levels of lighting when connected to a three-way switch. Instead the standard compact fluorescent lamp can be modified to include two different arc tubes in parallel to the lamp base, where each of the arc tubes operates at full lumen output when activated by the three-way switch. This dual arc tube solution requires two ballasts, one to drive each of the arc tubes.